Egyptians mourn over the bodies of their relatives in the El-Iman mosque in Nasr City, Cairo. Egypt faces a phase of uncertainty after the bloodiest day since its Arab Spring began.

Photo: Khalil Hamra, Associated Press

Egyptians mourn over the bodies of their relatives in the El-Iman...

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A man sits beside a coffin, mourning those who died during clashes between supporters of ousted Egyptian president Mohammed Morsi and security forces in the Nasr City neighborhood of Cairo, Aug. 15, 2013. The death toll from Egyptâ€™s bloody crackdown on supporters of the deposed president soared beyond 500 Thursday with more than 3,700 people injured, the Health Ministry said. (Tara Todras-Whitehill/The New York Times)

Photo: Tara Todras-whitehill, New York Times

A man sits beside a coffin, mourning those who died during clashes...

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Palestinians hold light candles in front of their national and Egyptian flags and leaflets during a silent protest organized by Palestinian Youths Organization, link to Hamas, supporting Egypt's ousted President, Mohammed Morsi, in Gaza City, Thursday, Aug. 15, 2013. Egyptian authorities on Thursday authorized police to use deadly force to protect themselves and key state institutions from attacks, after presumed supporters of the deposed Islamist president torched two local government buildings near the capital in the latest of a series of apparent reprisals to follow a bloody crackdown on their protest camps. Arabic on leaflets reads, "Stop the massacre in Egypt." (AP Photo/Adel Hana)

Photo: Adel Hana, Associated Press

Palestinians hold light candles in front of their national and...

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Egyptians carry the corpse of a supporter of ousted Egypt's Islamist President Mohammed Morsi in the El-Iman mosque at Nasr City, Cairo, Egypt, Thursday, Aug. 15, 2013. Egypt faced a new phase of uncertainty on Thursday after the bloodiest day since its Arab Spring began, with hundreds of people reported killed and thousands injured as police smashed two protest camps of supporters of the deposed Islamist president. Wednesday's raids touched off day-long street violence that prompted the military-backed interim leaders to impose a state of emergency and curfew, and drew widespread condemnation from the Muslim world and the West, including the United States. (AP Photo/Manu Brabo)

Photo: Manu Brabo, Associated Press

Egyptians carry the corpse of a supporter of ousted Egypt's...

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Yemeni protesters gather in solidarity with the Muslim Brotherhood and ousted Egyptian president Mohammed Morsi in a rally in Sanaa on August 15, 2013. Egypt's Tamarod group, which launched the call for mass protests that led to the ousting of Islamist president Mohamed Morsi, urged all Egyptians to take to the streets on Friday to defend the country from "terrorism". AFP PHOTO/ MOHAMMED HUWAISMOHAMMED HUWAIS/AFP/Getty Images

Photo: Mohammed Huwais, AFP/Getty Images

Yemeni protesters gather in solidarity with the Muslim Brotherhood...

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Yemeni protesters gather in solidarity with the Muslim Brotherhood and ousted Egyptian president Mohammed Morsi in a rally in Sanaa on August 15, 2013. Egypt's Tamarod group, which launched the call for mass protests that led to the ousting of Islamist president Mohamed Morsi, urged all Egyptians to take to the streets on Friday to defend the country from "terrorism". AFP PHOTO/ MOHAMMED HUWAISMOHAMMED HUWAIS/AFP/Getty Images

Photo: Mohammed Huwais, AFP/Getty Images

Yemeni protesters gather in solidarity with the Muslim Brotherhood...

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A Yemeni youth holds a poster of Egypt's ousted President Mohammed Morsi after climbing a lamp post during a rally supporting Morsi in Sanaa, Yemen, Thursday, Aug. 15, 2013. Arabic writing reads, "We are with the legitimacy." Egyptian authorities on Thursday authorized police to use deadly force to protect themselves and key state institutions from attacks, after presumed supporters of the deposed Islamist president torched two local government buildings near the capital in the latest of a series of apparent reprisals to follow a bloody crackdown on their protest camps. (AP Photo/Hani Mohammed)

Photo: Hani Mohammed, Associated Press

A Yemeni youth holds a poster of Egypt's ousted President Mohammed...

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A Yemeni protester, center, wearing posters of Egypt's ousted President Mohammed Morsi over his head, chants slogans during a rally supporting Morsi in Sanaa, Yemen, Thursday, Aug. 15, 2013. Arabic writing reads, "We are with the legitimacy." Egyptian authorities on Thursday authorized police to use deadly force to protect themselves and key state institutions from attacks, after presumed supporters of the deposed Islamist president torched two local government buildings near the capital in the latest of a series of apparent reprisals to follow a bloody crackdown on their protest camps. (AP Photo/Hani Mohammed)

AP10ThingsToSee - A supporter of ousted Islamist President Mohammed...

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A man mourns over the body of a relative at the El-Iman mosque, where the bodies of pro-Morsi protesters who were killed in Wednesday's government crackdown were brought for identification, in the Nasr City district of Cairo, Aug. 15, 2013. The death toll from Egyptâ€™s bloody crackdown on supporters of ousted President Mohammed Morsi soared beyond 500 Thursday, with more than 3,700 people injured, the Health Ministry said. (Bryan Denton/The New York Times)

Photo: Bryan Denton, New York Times

A man mourns over the body of a relative at the El-Iman mosque,...

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A soldier guards a doorway to the Rabaah al-Adaqiya mosque, which was destroyed during clashes between supporters of ousted Egyptian president Mohammed Morsi and security forces in the Nasr City neighborhood of Cairo, Aug. 15, 2013. The death toll from Egyptâ€™s bloody crackdown on supporters of the deposed president soared beyond 500 Thursday with more than 3,700 people injured, the Health Ministry said. (Bryan Denton/The New York Times)

Photo: Bryan Denton, New York Times

A soldier guards a doorway to the Rabaah al-Adaqiya mosque, which...

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Egyptian civilians help firefighters to battle flames at the Giza governorate building that was stormed and torched by angry supporters of Egypt's ousted president, Cairo, Egypt, Thursday, Aug. 15, 2013. Egypt faced a new phase of uncertainty on Thursday after the bloodiest day since its Arab Spring began, with hundreds of people reported killed and thousands injured as police smashed two protest camps of supporters of the deposed Islamist president. Wednesday's raids touched off day-long street violence that prompted the military-backed interim leaders to impose a state of emergency and curfew, and drew widespread condemnation from the Muslim world and the West, including the United States. (AP Photo/Hassan Ammar)

Photo: Hassan Ammar, Associated Press

Egyptian civilians help firefighters to battle flames at the Giza...

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TOPSHOTS Residents walk through debris and rubble inside the burnt down mosque of Rabaa al-Adawiya on August 15, 2013 in Cairo, Egypt. Islamists vowed to rally later in the day in support of deposed president Mohamed Morsi despite a violent crackdown that sparked Egypt's worst day of violence for decades, with over 500 people killed. As the death toll from the carnage soared, condemnation of the previous day's crackdown on two Muslim Brotherhood protest camps in Cairo poured in, with Britain, France and Germany summoning the country's ambassadors to express concern. AFP PHOTO/KHALED DESOUKIKHALED DESOUKI/AFP/Getty Images

Photo: Khaled Desouki, AFP/Getty Images

TOPSHOTS Residents walk through debris and rubble inside the burnt...

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CAIRO, EGYPT - AUGUST 15: An Egyptian woman identifies the body of a family member, a supporter of deposed Egyptian President Mohammed Morsi killed during a violent crackdown by Egyptian Security Forces on pro-Morsi sit-in demonstrations the day before, at the al-Iman Mosque in Nasr City on August 15, 2013 in Cairo, Egypt. An unknown number of pro-Morsi protesters were killed in Egypt's capital yesterday as Egyptian Security Forces undertook a planned operation to clear Morsi supporters from two sit-in demonstrations in Cairo where they have camped for over one month. Egyptian Police and Army forces entered protest sites in the Nasr City and Giza districts at dawn on August 14, using tear gas, live fire and bulldozers to disperse protesters and destroy the camps. A state of emergency has been declared in Egypt that began yesterday afternoon and will last for one month. (Photo by Ed Giles/Getty Images)

Photo: Ed Giles, Getty Images

CAIRO, EGYPT - AUGUST 15: An Egyptian woman identifies the body of...

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Egyptian Army soldiers walk among the charred remains of the Rabaah al-Adawiya mosque, in the center of the largest protest camp of supporters of ousted President Mohammed Morsi, that was cleared by security forces, in the district of Nasr City, Cairo, Egypt, Thursday, Aug. 15, 2013. Egypt faced a new phase of uncertainty on Thursday after the bloodiest day since its Arab Spring began, with hundreds of people reported killed and thousands injured as police smashed two protest camps of supporters of the deposed Islamist president. Wednesday's raids touched off day-long street violence that prompted the military-backed interim leaders to impose a state of emergency and curfew, and drew widespread condemnation from the Muslim world and the West, including the United States. (AP Photo/Hassan Ammar)

Photo: Hassan Ammar, Associated Press

Egyptian Army soldiers walk among the charred remains of the Rabaah...

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Egyptians mourn over the bodies of their relatives in the El-Iman mosque at Nasr City, Cairo, Egypt, Thursday, Aug. 15, 2013. Egypt faced a new phase of uncertainty on Thursday after the bloodiest day since its Arab Spring began, with hundreds of people reported killed and thousands injured as police smashed two protest camps of supporters of the deposed Islamist president. Wednesday's raids touched off day-long street violence that prompted the military-backed interim leaders to impose a state of emergency and curfew, and drew widespread condemnation from the Muslim world and the West, including the United States. (AP Photo/Khalil Hamra)

Photo: Khalil Hamra, Associated Press

Egyptians mourn over the bodies of their relatives in the El-Iman...

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An Egyptian holds a burned page form the Quran walk among the burned remains of the Rabaah al-Adawiya mosque, in the center of the largest protest camp of supporters of ousted President Mohammed Morsi, that was cleared by security forces, in the district of Nasr city, Cairo, Egypt, Thursday, Aug. 15, 2013. The death toll keeps going up in Egypt after security forces swept through two sit-in sites yesterday, operated by supporters of Morsi. An Egyptian Health Ministry spokesman now says hundreds of people died in the violence that has prompted international criticism. (AP Photo/Ahmed Gomaa)

As the death toll soared past 600 on Thursday, weeping relatives in search of loved ones uncovered the faces of the bloodied dead in a Cairo mosque near the flattened center of Islamist support for ousted President Mohammed Morsi.

Violence also spread, with government buildings set afire near the Pyramids, police officers gunned down and scores of Christian churches attacked.

As turmoil engulfed the country, the Interior Ministry authorized the use of deadly force against protesters targeting police and state institutions after Islamists torched government buildings, churches, police stations and cut main roads in retaliation for the crackdown on their encampments.

The Muslim Brotherhood, meanwhile, tried to regroup after the encampments were razed Wednesday and many leaders were arrested, calling for a mass rally on Friday in a challenge to the government's declaration of a monthlong nationwide state of emergency and a dawn-to-dusk curfew.

At least 638 people were confirmed killed and nearly 4,000 wounded in violence sparked when riot police backed by armored vehicles and bulldozers smashed the two sit-ins in Cairo, where Morsi's mainly Islamist supporters had been camped out for six weeks calling for his reinstatement. It was the deadliest day by far since the 2011 popular uprising that toppled autocratic ruler Hosni Mubarak and plunged the country into more than two years of instability.

The Health Ministry said that 288 of those killed were in the largest protest camp in Cairo's Nasr City district, while 90 others were slain in a smaller encampment at al-Nahda Square, near Cairo University. Others died in clashes that broke out between Morsi's supporters and security forces elsewhere in the Egyptian capital and other cities.

Morsi's Muslim Brotherhood put the casualty toll at a staggering 2,600 killed and 10,000 injured, but the figures appeared high in light of footage by regional and local TV networks, as well as the Associated Press.

The deadly crackdown drew widespread condemnation from the Muslim world and the West.

President Obama canceled joint U.S.-Egypt military exercises, although he gave no indication that the U.S. planned to cut off its $1.3 billion in annual military aid to the country. The U.S. administration has avoided declaring Morsi's ouster a coup, which would force it to suspend the military aid.

"While we want to sustain our relationship with Egypt, our traditional cooperation cannot continue as usual when civilians are being killed in the streets and rights are being rolled back," he said, speaking from his weeklong vacation in Massachusetts.

The president also ordered his national security team to "assess the actions taken by the interim government and further steps that we may take as necessary with respect to the U.S.-Egyptian relationship."